Data transfer between different data transfer devices can be arranged so that the data transfer devices, between which information is to be transferred, are connected for the time needed for data transfer. This so-called circuit switched connection is maintained until the user stops the data transfer. In these cases, most of the connection time is used for feeding the commands given by the user, and only a small part of the time is used for actual data transfer. This limits, among other things, the maximum number of simultaneous users of the same application. Another possibility is to use so-called packet-switched data transfer. This means that information is transferred between data transfer devices in packet form, whereby the time between packets is freely available for other data transfer devices. In that way, the number of simultaneous users can be increased especially in wireless data transfer networks, such as cellular networks, because then the mobile stations in the area of the same cell can use the same transmission channel. A well known cellular communication system is the GSM system, for which a packet transmission service called GPRS (General Packet Radio Service) has been developed. The blocks that are of central importance for the operation of the GPRS system are shown as a block diagram in FIG. 1. The Serving GPRS Support Node (SGSN) controls the operation of the packet switching service on the side of the cellular network. The serving GPRS support node takes care of the attachment to and detachment from the network of the mobile-station 2, updating the location of the mobile station 2 and directing the data packets to the right addresses. The mobile station 2 is connected to the base station subsystem BSS via a radio interface Um (FIG. 1). The base station system is connected to the serving GPRS support node SGSN via a BSS-SGSN interface Gb. In the base station subsystem BSS, the base transceiver station BTS and the base station controller BSC are interconnected by a BTS-BSC interface called Abis. The serving GPRS support nodes SGSN can communicate with other serving GPRS support nodes SGSN by means of the Gateway GPRS Support Node (GGSN).
The operation of both the mobile station 2 and the serving GPRS support node SGSN can be divided into several layers, each of which has a different function, as shown in FIG. 2.
The transfer of information, such as control signalling and information sent by the user, between the mobile station 2 and the serving support node SGSN preferably takes place in the form of data frames. The data frame of each layer consists of a header field and a data field. FIG. 2 also shows the structure of the data frames used in the GPRS system in different layers.
The information contained by the data field can be, for example, information fed by the user of the mobile station or signalling information. The functional tasks of the layers of the GPRS system are shown in the following.
Lowest in the Data Link Layer is the MAC (Media Access Control) layer, which takes care of using the radio path in communications between the mobile station 2 and the Base Station Subsystem), such as allocating the channels for transmission and reception of packets.
A time division/frequency division multiple access (TDMA/FDMA) method according to the GSM system is used in the physical layer (radio path) in the GPRS system. The basic transmission unit is called a burst, which consists of a certain number of bits sent to the radio path. The length of a burst is 15/26 ms, or approx. 0.577 ms. The radio path is also divided into channels, in which the difference between the medium frequencies is 200 kHz in the GPRS/GSM system.
Eight bursts or time slots constitute one TDMA frame. These frames are combined to form a larger 52-multiframe, which comprises 52 TDMA frames. FIG. 5a illustrates the structure of such a multiframe as a simplified diagram. These multiframes are used in the implementation of logical channels. The multiframe is divided into 12 radio blocks (RB0–RB11), each of which includes four TDMA frames. One radio block is needed to send one RLC/MAC frame. In addition, the multiframe includes two so-called search frames (S), during which the base station of the cell does not send on the logical packet data channel (PDCH). Then the mobile stations in the area of the cell can perform measurements of the signal strength and interference of the base stations of the adjacent cells. The remaining two frames are reserved for the transmission (T) of the Packet Timing Advance Channel (PTACH).
Logical channels have been formed in the GPRS system for various purposes of signalling and transmission of data packets. Of these logical channels the following may be mentioned in this context: PCCCH (Packet Common Control Channel), PBCCH (Packet Broadcast Control Channel), PDTCH (Packet Data Traffic Channel), PACCH (Packet Associated Control Channel) and PTACH (Packet Timing Advance Channel). The PCCCH channel is used, for instance, during a packet-switched connection to request transmission periods from the base station for the transmission of packets, to inform of the granted periods to the mobile station, to send search messages etc. In the PBCCH channel, the base station sends system information of the packet system to the mobile-station. The transmission of data packets is carried out in the PDTCH channel. The PACCH channel is used for the transmission of signalling information related to the transfer of packets (acknowledgements, measurement information and reports). The PTACH channel is used in connection with timing for evaluating different delays.
In the transmission stage, the bits formed from the packets, possibly coded, are not transferred to the bursts as such, but they are interleaved and matched at first. By interleaving, the bits are divided into four bursts so that bits of one byte % are not all placed in the same burst, but preferably in all four bursts in a certain order. This order of interleaving is known in the receiver, and thus it is possible to return the order of the bits. The purpose of matching is to place the interleaved bits in the desired points in each burst so that bits remain in the bursts for other purposes (stealing bits), such as for transmitting channel coding information or other control information.
Data transfer on the lowest level between the base station subsystem BSS and the serving GPRS support node is carried out in the L2 layer (data link layer), in which a link layer protocol, such as the known LAPD protocol, frame relay protocol or the like is used. The L2 layer can also include quality or routing information according to the GPRS specifications. The L2 layer has properties of the physical layer and data link layer of the OSI model.
Above the MAC layer, there is the RLC layer (Radio Link Control), the purpose of which is to divide the data frames formed by the LLC layer into packets of a certain length that can be sent to the radio path (PDU, Protocol Data Unit), to transmit them and retransmit them, when necessary. The length of the packets in the GPRS system is the length of one GSM time slot (approx. 0.577 ms).
The LLC layer (Logical Link Control) offers a reliable data transfer link between the mobile station 2 and the serving GPRS support node SGSN. Among other things, the LLC layer adds error check information to the message to be transmitted. Based on this information, incorrectly received messages can be attempted to correct, and the message can be retransmitted, when necessary. In addition, the encryption and decryption of the information is carried out in the LLC layer.
The functions carried out in the SNDCP layer (Sub-Network Dependent Convergence Protocol) include protocol changes of the information to be transmitted, compressing, segmentation and the segmentation of messages coming from an upper level. FIG. 2 also shows the structure of a SNDCP frame. The SNDCP frame comprises a SNDCP header field and a SNDCP data field. The SNDCP header field consists of protocol information (Network Service Access Point Identity, NSAPI) and SNDCP control information, such as compressing, segmentation and encryption specifications. The SNDCP layer functions as a protocol adapter between the protocols used on the upper level and the protocol of the LLC layer (data link layer).
The information to be transmitted comes preferably as data packets according to a suitable protocol, such as Packet Data Protocol (PDP), to the SNDCP layer from an application, like messages according to the X.25 protocol or the Internet protocol (IP). The application can be, for example, a data application of a mobile station, a telefax application, a computer program, which has a data transfer connection to a mobile station, etc.
The SNDCP frame is transferred to the LLC layer, where an LLC header field is added to the frame. The LLC header field consists of, for example, the LLC control part, which defines the number of the frame and the type of the command (info, acknowledgement, retransmission request, etc.) In connection with the attachment to the GPRS packet network, the mobile station sends an attach request message to the serving GPRS support node. On the basis of the International Mobile Station Identity (IMSI) of the mobile station, the serving GPRS support node can retrieve information from the home location register HLR corresponding to the mobile station in question, whereby the serving GPRS support node can use this information to select a Temporary Logical Link Identity (TLLI) for a data transfer connection. If the mobile station has used a TLLI before, it can transmit the TLLI in a request message, whereby the serving GPRS support node can give this TLLI to the mobile station again, or allocate a new TLLI to it. The serving GPRS support node SGSN transmits the selected TLLI to the mobile station for use in a data transfer connection between the mobile station and the serving GPRS support node. This TLLI is used in data transfer to determine to which data transfer connection each message belongs. The same TLLI may not be in use simultaneously in more than one data transfer connection. When the connection has terminated, the TLLI used in the connection can be given to a new connection to be established.
The operator of the packet-switched network has divided the cells of the packet-switched network into Routing Areas, which can be used in determining the location of the mobile station 2. Each routing area comprises one or several cells. Then the Mobility Management operations of the mobile station are used to keep record of the location and connection status of the mobile stations in the operation area of the packet-switched network. These records are maintained both in the mobile station and the packet-switched network, preferably in the serving GPRS support node SGSN. In the GSM system, the base station, which has a communications connection to the mobile station 2, is changed in connection with the reselection of a cell.
When a mobile station 2 is synchronized to the transmission of a cell in connection with start-up or when a mobile station moves to the area of another cell, the base station BTS sends information concerning, for example, the way that in which the logical channels in the area of the cell in question are arranged in physical channels, or in which radio block and time slot of the multiframe information of each logical channel is transmitted.
The packet system divides the mobile stations 2 in the area of the cell into so-called paging groups. Paging messages are then sent to each paging group in a certain radio block of the PCCCH channel. In the GPRS system, the division into paging groups is advantagely based on the International Mobile Subscriber Identity, the number of paging channels available in a cell and the number of paging blocks available in the paging channel.
In Code Division Multiple Access (CDMA) based cellular networks it is possible to communicate to the mobile station 2 via several base stations simultaneously. The base stations transmit a spread spectrum signal on a so-called pilot channel, whereby a mobile station can conclude on the basis of these pilot signals, which base station sends the best signal for communication. The base stations which are currently communicating with the mobile station 2 form a so-called active set. The movement of the mobile station by means of the packet-switched network of the CDMA system can be concluded from the changing of these active sets.
A mobile station in a GPRS system can have, among other things, the three following connection modes in relation to the packet-switched network: an idle mode (IDLE), a standby mode (STANDBY) and an active mode (READY). In the idle mode, the mobile station is not connected to the mobility management of the network and in which case the mobile station is not able to communicate with the network. The mobility management information of the mobile station 2 and the mobility management information of the serving GPRS support node concerning the mobile station 2 in question is not necessarily up to date, if the mobile station has moved to the area of another cell while in the idle mode. When required, the mobile station 2 performs the selection and reselection of a cell in the Public Land Mobile Network (PLMN) and the packet-switched network. In relation to the packet-switched network, a mobile station 2 in the idle mode is not connected to the network
In the active mode the mobile station is connected to the mobility management of the packet-switched network, the location of the mobile station is known in the packet-switched network within the accuracy of a cell, and the mobile station can both send and receive data packets. The selection and reselection of a cell in the packet-switched network is performed by either the mobile station 2, or the packet-switched network can control the selection of a cell. The header field of the Base Station Subsystem GPRS Protocol (BSSGP) packet includes the cell identification information. In a system based on the packet-switched network described here the purpose of the GPRS protocol level of this base station subsystem is to transmit information related to the routing and the Quality of Service (QoS) between the Base Station Subsystem (BSS) and the serving GPRS support node SGSN.
In the active mode, the mobile station is connected to the mobility management of the packet-switched network, but the mobile station cannot send or receive data packets. The location of the mobile station in the packet-switched network is known only within the accuracy of the routing area. The reception of paging requests from the serving GPRS support node for cell selection, (CS) services is possible. Changing from the active mode to the standby mode can be done when, for example, a sufficiently long time has passed since the transmission of the last data packet between the mobile station and the packet-switched network. A mobile station 2 in the standby mode can start either activation or deactivation of a packet-switched connection (PDP). The status of the packet-switched connection must be updated before sending or receiving data packets. If a packet-switched connection has been activated, the serving GPRS support node can receive packets. Then the serving GPRS support node SGSN sends a paging request to the routing area where the mobile station 2 is located. When the mobile station 2 sends a reply message to this request, the connection mode of the mobile station 2 is changed into the active mode. The connection mode of the serving GPRS support node SGSN is changed into the active mode after the serving GPRS support node SGSN has received a reply message to a paging request from the mobile station 2. The connection mode of the mobile station 2 is changed from the standby mode to the active mode also when the mobile station sends data packets or signalling information to the serving GPRS support node SGSN. In a corresponding manner, in the serving GPRS support node SGSN the change of the connection mode from the standby mode to the active mode is also done in the situation in which the SGSN receives data packets sent by the mobile station 2, or the SGSN receives signalling information.
The mobile station receives paging messages from the base station which the mobile station is listening to at the time. The mobile station can find out from these paging messages whether there are transmissions coming to it from the base station. The time between two consecutive paging messages is called a DRX period (discontinuous reception). Later in this specification, this DRX period will be called a paging period. During a paging period, the mobile station can set itself to a power saving mode for a certain time, because it does not expect to be getting any transmissions from the mobile communication network. Such a discontinuous reception mode (DRX mode) is allowed for a mobile station, which is in a standby mode in all other times except when the mobile station is performing cell selection functions. The length of the period can vary, and the mobile station receives parameter information from the base station. The mobile station can calculate on the basis of the received parameter information when the next paging message can be expected. Because the mobile station is synchronized to the base station transmission, the mobile station knows the transmission time of the next paging message. A discontinuous reception mode like this enables switching all possible functional blocks connected with the radio interface off when they are not needed. Functional blocks like this are, for example: the radio part, the baseband part, which preferably also comprises a digital signal processing unit, and a system oscillator used in the radio interface functions. The purpose of this arrangement is to reduce the overall power consumption of the mobile station.
However, the mobile station must change from the idle mode back to the standby mode at times for receiving the above mentioned paging messages, for example. In the GPRS system, the maximum length of the idle mode is specified as a time corresponding to 64 52-multiframes, or approx. 15 s. Then the maximum delay for establishing a connection with the mobile station is approx. 15 seconds. However, in practical applications the maximum length of the idle state is made much shorter, nine 52-multiframes, or approx. two seconds, because of the specifications of the GSM system and the fact that the frequency stability of an oscillator used in the idle state may not be sufficient to keep the mobile station synchronized to the mobile communication network. If a prior art mobile station does not keep synchronization to the mobile communication network during the idle mode, this can mean that the mobile station cannot receive the next paging message. On the other hand, the power consumption of a mobile station is generally the higher the shorter the idle period is.